Solid-state terahertz devices could scan for cancer

Electron microscope image of a prototype chip using a ring of coupled oscillators to generate terahertz radiation. Silicon cannot oscillate in the terahertz range, but the design focuses most of the energy in a high harmonic. The signal radiates on the axis of the ring and can be aimed. Image: Ehsan Afshari

Cornell University researchers have developed a new method of generating
terahertz signals on an inexpensive silicon chip, offering possible
applications in medical imaging, security scanning, and wireless data transfer.

Terahertz radiation, the portion of the electromagnetic spectrum between
microwaves and infrared light, penetrates cloth and leather and just a few
millimeters into the skin, but without the potentially damaging effects of
X-rays. Terahertz scanning can identify skin cancers too small to see with the
naked eye. Many of the complex organic chemicals used in explosives absorb
terahertz radiation at particular frequencies, creating a "signature"
that detectors can read. And because higher frequencies can carry more
bandwidth, terahertz signals could make a sort of super Bluetooth that could
transfer an entire high-definition movie wirelessly in a few seconds.

Current methods of generating terahertz radiation involve lasers, vacuum
tubes, and special circuits cooled near absolute zero, often in room-sized
apparatus costing thousands of dollars. Ehsan Afshari, assistant professor of
electrical and computer engineering, has developed a new method using the
familiar and inexpensive CMOS chip technology, generating power levels high
enough for some medical applications. With further research, higher power will
be possible, Afshari said, enabling such devices as handheld scanners for law
enforcement.

Afshari and graduate students Yahya Tousi and Vahnood Pourahma describe the
new approach in Physical Review Letters.

The ability of solid-state devices to generate high frequencies is limited
by the characteristics of the material—basically, how fast electrons can move
back and forth in a transistor. So circuit designers make use of harmonics—signals
that naturally appear at multiples of the fundamental frequency of an
oscillator. That fundamental frequency is usually set by a circuit that uses a
variable capacitor called a varactor, but at terahertz frequencies varactors
don't tune sharply. Afshari has come up with a new way of tuning by coupling
several oscillators in a ring, producing what engineers call a high-quality signal,
where all the power goes into a very narrow frequency band.

Connect two springs and set one vibrating, and the other will begin to
vibrate as well, and eventually they will settle to an equilibrium. A ring of
electronic oscillators does the same, and the circuits coupling the oscillators
can set the frequency at which they will lock in. In Afshari's device the
couplers also shift the phase of the signals, that is, how the peaks and
valleys of the waves line up. With the right adjustment, the peaks and valleys
cancel each other out at several harmonics but reinforce each other at one—in
this case the fourth—channeling most of the power there.

In early experiments, the researchers fabricated chips that generated
signals with about 10,000 times the power level previously obtained at
terahertz frequencies on a silicon chip. The signal emerges along the axis of
the ring, and what the researchers called an intriguing possibility is that by
adjusting the couplers separately they could aim the output, making it possible
to scan large areas with a narrow, high-powered beam.

The power could be increased by adding more oscillators to the ring or using
multiple rings, and Afshari is working with Cornell experts on gallium nitride,
a chip material that can handle both higher frequencies and higher power. But
Afshari said he wants to focus on less-expensive silicon. "The goal is to
make a complete device on one CMOS chip," he said. "I can envision a
tiny thing you could put in a cell phone."